1. Field of Invention
The present invention relates to a memory cell and fabrication process thereof, and more particularly to a memory cell suitable for fabricating a Metal-Oxide-Nitride-Oxide-Poly-Silicon (MONOS) on a substrate.
2. Description of Related Art
Since the liquid crystal display (LCD) and the organic light emitting diode (OLED) display have the advantages of being light, thin, short, and small, they have gradually become the display for the portable terminal system. Especially, the twisted nematic liquid crystal display (TN-LCD), super twisted nematic liquid crystal display (STN-LCD), and thin film transistor liquid crystal display (TFT-LCD) have become the indispensable display products. The TFT-LCD pixel mainly includes a thin film transistor, a storage capacitor, and a pixel electrode. The image data written in each pixel is stored in the storage capacitor and is updated frame by frame. Therefore, the power consumption of the TFT-LCD having this architecture is quite high.
As for most of the current portable electronic devices, the LCD is used to display static images for most of the time, so it is unnecessary to keep updating the image data stored in the pixel. In this case, if a memory, such as SRAM or DRAM, is embedded in each pixel, the power consumption of the LCD can be greatly reduced.
FIG. 1 is a circuit diagram of a conventional pixel structure. Referring to FIG. 1, a conventional pixel structure 100 for displaying a static picture includes a thin film transistor 110, a liquid crystal capacitor 120, a memory control circuit 130, and a SRAM 140. The gate G of the thin film transistor 110 is electrically connected to the scan line SL. The source S of the thin film transistor 110 is electrically connected to the data line DL. The drain D of the thin film transistor 110 is electrically connected to the liquid crystal capacitor 120. In addition, the drain D of the thin film transistor 110 can be electrically connected to the SRAM 140 through the memory control circuit 130, such that the image signal input to the liquid crystal capacitor 120 from the data line DL can be stored in the SRAM 140 through the memory control circuit 130.
In the circumstance of displaying static images, the SRAM 140 can keep a voltage difference between two electrodes of the liquid crystal capacitor 120 without updating the data continuously, and thus the power consumption can be significantly reduced. However, the SRAM 140 is composed of at least four thin film transistors T1, and the memory control circuit 130 is composed of at least two thin film transistors T2. The thin film transistors T1, T2 make the circuit layout of the pixel structure 100 quite crowded, and adversely affects the aperture ratio of the pixel structure 100. Therefore, the pixel structure 100 can only be used in the reflective LCD panel instead of the transmissive LCD panel.